This invention relates to a method for operating a photo-to-photo transducer (abbreviated as PPC hereinafter) which transduces information carried by an electro-magnetic radiation beam such as light into the same form applicable to an image pickup apparatus, an optical projector or an optical computer.
A video signal obtained by picking up an optical image of an object can be easily processed by editing, trimming and the like. Recording, reproducing and erasing the video signal also can be easily performed. These techniques have been widely used in various fields such as printing, electronic publishing, measuring and the like.
There has been an increased demand for an apparatus capable of imaging and recording an optical information such as a moving picture or an image with a higher resolution when compared to a conventional apparatus.
In order to reproduce images with high quality and resolution, it is necessary for an image pickup apparatus to generate a video signal by which an image of high quality and resolution can be reproduced. However, an image pickup apparatus using an image pickup tube has a limit for minimizing the diameter of an electron beam, and also the target capacity thereof increases as its size becomes large, resulting in a poor resolution.
Furthermore, for high resolution moving images, the frequency band of a video signal becomes larger than several tens to hundreds MHz, thus posing a problem of poor S/N. The above problems have made it difficult to obtain video signals which can reproduce images of high quality and resolution.
As for a PPC capable of receiving an optical image of an object and emitting the optical image applicable to optical parallel processing in an optical projector and an optical computer and image-recording, for example, a space-modulation device such as a liquid crystal-type optical modulator, a photoconductive-Pockels effect device and a microchannel-type optical modulator or a device made of a photochromic material have been remarked on.
The assignee of this application has proposed an image pickup apparatus with high resolution employing the PPC which is shown in FIG. 1.
Throughout the drawings, like reference numerals and letters are used to designate like or equivalent elements for the sake of simplicity of explanation.
The image pickup apparatus shown in FIG. 1 comprises an imaging lens 10, a PPC 12, a beam splitter 24, a wave-plate 26, an analyzer 28, a polarizer 30 and a light source 32 such as a semiconductor laser.
The PPC 12 is composed of transparent electrodes 14 and 22, a photoconductive layer (abbreviated as PCL) member 16, a dielectric mirror 18 and a photo-modulation layer (abbreviated as PML) member 20 which is an optical member (such as a monocrystal of lithium niobate or a field effect nematic liquid crystal layer) for polarizing a plane of light according to with the intensity of an applied electric field. A power supply 34 causes an electric field across the transparent electrodes 14 and 22 by turning on a switch 36. The PPC 12 is installed into a black box not shown so that the PCL member 16 is not unnecessarily exposed to ambient light.
The transparent electrode 14 is composed of a material such as ITO (Indium-Tin Oxide) having a spectral characteristic such that light having a wave-length carrying optical information to be photographed passes therethrough.
As for the PCL 16, a photoconductive material (such as amorphous silicon) having a characteristic such that a field intensity distribution with high resolution is generated on a surface thereof when an optical image with high resolution is incident to the other surface thereof under an electric field with proper intensity.
In FIG. 1, light carrying an optical image of an object O is incident to the transparent electrode 14 through an imaging lens 10. The light passes through the transparent electrode 14 and is incident to the PCL member 16. The electric resistance of the PCL member 16 varies accordingly with the intensity of the light. The electric resistance of each portion of the PCL member 16 thus varies accordingly with the intensity of each portion of the object O.
The power supply 34 applies a specific voltage across the transparent electrodes 14 and 22 through the switch 36. Therefore, the electric field (a voltage Vm) with a field intensity corresponding to the optical image is applied across the PML member 20 which is provided so as to face the PCL member 16 through the dielectric mirror 18.
If the PML member 20 is composed of monocrystal of lithium niobate, the refractive index thereof varies with the electric field due to the electro-optic effect. Therefore, in the PML member 20, the refractive index varies with the field intensity distribution based on the impedance of the PCL member 16 varying accordingly with the optical image incident thereto.
A light P1 such as a laser beam emitted from the light source 32 passes through the polarizer 30 and is reflected at the beam splitter 24, then is incident to the PML member 20 through the transparent electrode 22.
The electric field whose field intensity-distribution is varying accordingly with the optical image is generated across the PML member 20, so that the light P1 thus incident thereto is modulated accordingly with the optical image. The light P1 thus modulated is reflected at the dielectric mirror 18 and passes through the beam splitter 24, the wave-plate 26 and the analyzer 28, then is picked up as a reproduced image.
FIG. 2 shows the relationship between the primary electro-optic effect and the voltage Vm applied to the PML member 20 (composed of a material such as monocrystal of lithium niobate LiNbO.sub.3, Bismuth Silicon Oxide, Potassium dihydrogen phosphate and DKDP). The axis of ordinates denotes the intensity I of the light P1 after passing through the wave-plate 26 and the analyzer 28, and the abscissa denotes the voltage Vm applied to the PML member 20. FIG. 3 shows the same relationship as that shown in FIG. 2. In this case, the PML member 20 is made of field effect nematic liquid crystal.
Each of the waveforms shown in FIGS. 2 and 3 with time base in an ordinates direction is the voltage applied to the PML member 20 accordingly with the optical image of the object O. While each of those in an abscissa direction is the light emitted from the analyzer 28, whose intensity is varying with the voltage in the ordinates direction.
As is understood from FIGS. 2 and 3, setting the operating point of the PML member 20 is done by adjustment of the wave-plate 26 or reorientation of the analyzer 28, so that the black level of the optical information of the optical image of the object O is reproduced as it is.
However, in the case of employing scattering-type liquid crystal or PLZT (Lead Lanthanum Zirconate Titanate) of the scattering mode as the PML member 20, the operating point thereof cannot be set by adjustment of the wave-plate 26 or reorientation of the analyzer 28.
Furthermore, the image pickup apparatus proposed by the assignee of this application requires the analyzer 28 so that the utility efficiency of light is inevitably lowered or shading occurs.
Moreover, when a twisted-nematic liquid crystal or other electro-optic liquid crystal having a double refraction characteristic is employed as the PML member 20, the utility efficiency of light is substantially lowered when a light P1 is incident to the PML member from the direction other than a normal line on the surface of the PML member 20 since photo-modulation therein depends on the direction of an optical axis of the liquid crystal.
Therefore, the incidence direction of the light P1 to the PML member 20 is set in the normal direction. Optical members such as a semi-transparent mirror and a deflection-beam splitter are thus provided along with an optical passage so as to separate the light P1 incident to the PPC 12 is reflection type.
When a twisted-nematic liquid crystal layer is employed as the PML member 20, a complex process is required, that is, a twisted-nematic liquid crystal is injected into a cell made with spacers. Furthermore, when a large PPC is produced, it is difficult to compose a PML by means of a twisted-nematic liquid crystal layer with uniform thickness. Moreover, when a monocrystal of lithium niobate or other solid state component is employed as the PML member 20, a high electric field (or a voltage) is required for obtaining a desired electro-optic effect and the PPC 12 must be handled very carefully.
When the PPC 12 is composed flat, a complex deflection-optical system such as a doublet spatial lens is required for writing image information on the entire surface of the PPC 12.